The present invention relates to the field of aircraft propelled by at least one turbofan fastened in the vicinity of their fuselage, particularly below a wing or to the rear, and is aimed at a means for attenuating the noise emitted by the engine.
A turbofan consists of a gas turbine engine driving a cowled fan which is generally placed upstream. The mass of air sucked in by the engine is divided into a primary stream and a secondary stream, these streams being concentric. The primary stream is guided toward the primary body where it is compressed again, heated in a combustion chamber, guided toward successive stages of turbines and then ejected in a primary gas stream. The secondary stream is compressed by a cowled fan stage and then exhausted directly without having been heated. The two streams can be exhausted separately in two concentric streams or else mixed in a common duct prior to being exhausted. One or more turbine stages of the primary body are assigned to driving the fan. The jet engine is housed in a nacelle configured in such a way as to make the aerodynamic drag as small as possible. In the case of an engine in which the primary and secondary streams are exhausted separately, the nacelle comprises a first part encasing the fan and a second part forming the cowling of the primary body. The two casings are each terminated in the downstream direction by an exhaust nozzle, one for the primary stream and one for the secondary stream. In the case where the streams are mixed, the nacelle forms a single outer casing terminated by an exhaust nozzle for the combined streams: inside the nacelle the exhaust plane of the exhaust nozzle for the primary stream is situated upstream with respect to the exhaust plane for the mixed stream.
The noise emissions from civil turbofan engines have two main sources: the jet leaving the nozzle and the noise of the fan upstream. The present invention is concerned with the noise emitted by the jet leaving the nozzle.
The noise emitted by aircraft is a nuisance which the aviation industry is constantly seeking to reduce. This noise is particularly perceptible during the approach and takeoff phases in areas, usually urban areas, situated in the vicinity of airports. Many solutions have already been proposed including, in particular, that described by K. Viswanathan in the article AIAA 2004-2975 “An elegant concept for reduction of jet noise from turbofan engines”. A considerable noise reduction is observed in the rear direction of a bypass engine when the nozzle for the primary stream has an exhaust plane inclined in the upstream direction. This plane corresponds to the plane formed by the free edge of the nozzle. It is inclined with respect to the plane perpendicular to the axis of the engine by an angle which may range up to 45°. The axis of the engine and that of the gas stream correspond to one another.